Structural joints in aircraft applications frequently involve the joining of metallic and composite structures. These joints are accomplished using typical fastening concepts which suffer from significant strength reductions caused by the need to drill holes in the metallic member. This joining method also requires significant setup time in drilling holes and installing fasteners to attach the members to one another. Such holes often produce localized stresses and mechanical loads that the structure must account for. To account for such localized loads, the structures are typically reinforced resulting in increased weight and loads to be handled by the structure. Furthermore, quality assurance issues may arise when installing such fasteners (misdrilled holes and improper fastener installation is common).
Friction Stir Welding (FSW) is a newer joining method, as illustrated in FIG. 1 which has gained acceptance as a means for joining all metal panels together. FSW produces a plasticized region 22 of material by pushing a non-consumable rotating tool 24 into the material of parts 26A and 26B that are to be welded. Then a central pin, or probe, 28 followed by the shoulder 30, is brought into contact with the two parts 26A and 26B to be joined. The rotation of tool 24 heats up and plasticizes the materials that the tool is in contact with. As tool 24 moves along the joint line 32, material from the front of the tool is swept around this plasticized annulus to the rear, so eliminating the interface.
There are cost advantages if one applies a simple stiffened skin structure that may be produced via FSW to the exterior of a vehicle such as an aircraft. The robustness and automation of the process is very attractive for manufacturing. However, smaller complex three dimensional structures, such as aircraft designs, have not been easily addressed by the application of FSW. Furthermore, the FSW process works with metals as opposed to composite materials. The FSW process clamps two pieces that abut one another and then mixes the materials of the two pieces. This is most effectively achieved when two metallic pieces are forming a single two-dimensional surface. Thus it has been difficult to apply FSW processing to complex three-dimensional structures that involve both composites and metals.
Further limitations and disadvantages of conventional and traditional joining process and related structures and functionality will become apparent to one of ordinary skill in the art through comparison with the present invention described herein.